Just gone through my In Box on iPad they go back to 15.12.14.

For my information, do you know the difference between POP and IMAP, and do you know which one is being used by your iPad and Thunderbird?

I don't know, sorry.

With POP, all the e-mails from the server get sent to Thunderbird, Thunderbird saves those e-mails on your computer, then tells the server to delete those mails from the server. Unless you set an option in Thunderbird to not tell the server to delete the e-mails.

With IMAP, Thunderbird continually communicates with the server to show the e-mails on the server, but doesn't necessarily save them on your computer.

When using multiple devices to access the same e-mail, the only way to keep the e-mails synchronized between the devices is to use IMAP.

Another thing to remember, is that anything in the "Local Folders" in Thunderbird are only on your computer.

So probably what you want to do is Switch from POP to IMAP account in Thunderbird.

You can find instructions for doing the same thing for your iPad here: http://www.pop2imap.com/